1S65.] The Proposed Pendulum Operations for India. 271 



barometer are read by means of the catbetometer. It is usual to 

 observe not every coincidence, but tbe first three consecutive coin- 

 cidences, andtbentbe 11th, 12th, 13th, then the 21st, 22nd, 23rd, and 

 so on ; after observing the first tAvo or three, the times of the after 

 coincidences can be easily computed with sufficient accuracy to intimate 

 when the observer should be ready to note them. 



It is intended to have observations made generally along the Great 

 Arc at stations 2" apart in latitude, and at other points where it may 

 be desirable to obtain data regarding local variations in the intensity 

 of gravity. 



The pendulum experiments in this country will afford an independ- 

 ent value of the ellipticity of the Indian arc. It is also hoped that 

 they will throw some light on the existing discordances between the 

 astronomical and geodetic latitudes of the Indian survey. 



The amount of the deflections of the plumb line, due to the Hima- 

 layas and elevated table lands to the north of India, have been computed 

 by Archdeacon Pratt for the different terminal stations of the Indian 

 arcs ; but these determinations are so much in excess of the results of 

 the survey, that it is evident that the effects of the mountain attraction 

 must be in a considerable degree compensated, either by a deficiency 

 of density in the strata to the north, or by an excess of density in the 

 strata to the south of the survey stations. 



Now the peudulum can undoubtedly be made the means of showing 

 whether the compensation is to be attributed to either of these causes ; 

 for, whilst the effect of a distant range of mountains on the vibrations 

 would be quite inappreciable, any local variation in the density of the 

 underlying strata would show itself most unmistakably ; so that by 

 taking observations both at a normal station, and at a few points in 

 its vicinity symmetrically situated around it, should there be any con- 

 siderable excess or defect in the density of the strata to counteract 

 the disturbance due to the mountain mass, the pendulum observations 

 would not fail to point it out.* 



* Professor Stokes remarks in his letter on these opei'ations : " The peudu- 

 " rum no doubt indicates only the vertical component of the disturbing force, 

 cl whereas it is the horizontal component in the plane of the meridian that affects 

 " the measures of arcs ; at any one station, of course, a horizontal disturbance 

 " may exist without a vertical disturbance, and vice versa ; but in a system of 

 " stations disturbances of the one kind must necessarily bo accompanied by dis- 



